This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/DK98/00488 which has an International filling date of Nov. 11, 1988, which designated the United State of America.
The present invention relates to the catalytic oxidation of hydrocarbons to give useful oxidation products thereof. In particular, an important aspect of the invention relates to a process for the production of alkanols, e.g. lower alkanols, such as C1-C3 alkanols (e.g. methanol or ethanol), from the corresponding alkanes (i.e. methane and ethane, respectively, in the case of methanol and ethanol), wherein an alkanol is produced via the formation, and subsequent hydrolysis, of an intermediate (e.g. methyl bisulfate in the case of methane as starting hydrocarbon) formed by catalytic oxidation of the hydrocarbon in a liquid, essentially anhydrous sulfuric acid (H2SO4) medium containing a catalyst and preferably containing a significant proportion of dissolved sulfur trioxide (SO3).
Methane is a raw material of great synthetic importance and an abundant natural resource as the main constituent of natural gas. Nevertheless, it is primarily used only as fuel because two factors limit its use as a raw chemical. The first is that transporting methane gas or even liquefied natural gas is not economical. Therefore, it is highly desirable to transform methane into transportable raw materials or products. The second factor is that methane is a very stable molecule and its direct conversion to useful chemicals is very difficult. Today, over 90% of the produced methane is consumed as heating fuel. Because transportation of natural gas from remote sites is costly, it has often been suggested that natural gas, namely methane, should be converted to more easily transported liquid fuel.
As a raw material for chemical industries, the two leading uses of methane are in the production of methanol and ammonia. Methane must be first transformed into synthesis gas before usage in either ammonia or methanol synthesis. Clearly, this process makes conversion into synthesis gas the dominant process of methane upgrading. The term synthesis gas is generally used for a mixture of carbon monoxide and hydrogen, preferably at a 1:2 or 1:3 ratio. Today the dominant route to the production of synthesis gas is the methane steam reforming process. The reaction can be stoichiometrically expressed as
CH4+H2Oxe2x86x92CO+3H2
A considerable disadvantage of the steam reforming process is that it is an endothermic reaction. The endothermicity results from addition of steam in which a significant amount of energy is required to decompose water into its elements.
In addition to synthesis gas formation, several documents disclose a variety of methods for activating methane to produce other higher molecular weight materials. Mobil Oil Corporation is the assignee in several U.S. patents using sulfur or certain sulfur-containing compounds as the reactants in non-catalytic reactions with methane to produce methyl intermediates which can then be converted to higher molecular weight hydrocarbons.
In U.S. Pat. No. 4,543,434, Chang teaches a process using the following steps:
CH4+4Sxe2x86x92CS2+2H2S
CS2+3H2(Co or Ni)xe2x86x92CH3SH+H2S
CH3SH (HZSM-5)xe2x86x92[CH2]+H2S
4H2Sxe2x86x924H2+4S
where xe2x80x9c[CH2]xe2x80x9d is a hydrocarbon having at least two carbon atoms.
Another Mobil disclosure (U.S. Pat. No. 4,864,073 to Han et al.) suggests a carbonyl sulfide-based process in which methane and carbonyl sulfide are contacted in the presence of ultraviolet light under conditions sufficient to produce CH3SH. No other reaction initiators are said to be present. The reaction scheme is shown to be:
xe2x80x83CH4+COSxe2x86x92CH3SH+CO
CH3SH (HZSM-5)xe2x86x92[CH2]H2S
(H2Sxe2x86x92S), a regeneration step
CO+Sxe2x86x92COS
The selectivity of the first reaction is said to be high, i.e., around 81%, however, the conversion appears to be quite low.
A disclosure similar to that of Chang is found in Mobil""s U.S. Pat. No. 4,864,074 to Han et al. As in Chang, the methane is contacted with sulfur. The process conditions are changed, however, so that either CS2 or CH3SH is formed. These sulfur compounds may then be converted in the presence of the preferred HZSM-5 zeolite catalyst to produce hydrocarbons having two or more carbon atoms. Also, as was the case with xe2x80x9cChangxe2x80x9d, the step of contacting the methane to produce a methyl-sulfur compound is performed in the absence of a catalyst.
Other methods are known for producing substituted methanes which are suitable for further reaction to heavier hydrocarbons. A thermal methane chlorination process is shown in U.S. Pat. No. 4,804,797 to Minet et al. A similar process is disclosed in U.S. Pat. No. 3,979,470 to Fimhaber et al. (Although a preference for C3 hydrocarbon feeds is expressed in the patent).
One method shown in U.S. Pat. No.4,523,040 to Olah utilizes either a solid strongly acidic catalyst or a supported Group VIII metal (particularly platinum and palladium) in the gas phase halogenation of methane to produce methyl halides. The patent indicates that monohalides are produced with 85% to 99% selectivity. Olah suggests that subsequent or concurrent catalytic hydrolysis produces methyl alcohol and/or dimethyl ether. Production of methyl oxo-esters is not shown.
The reaction of methane with palladium (II) acetate in trifluoroacetic acid to effect the trifluoroacetoxolation of methane is shown in Sen et al., xe2x80x9cPalladium (II) Mediated Oxidative Functionalization of Alkanes and Arenesxe2x80x9d, New Journal of Chemistry (1 989), Vol. 13, No. 10-11, pp. 756-760. A yield of 60% based on palladium was reported when the reaction was practiced using methane as the reactant. Consequently, the reaction with methane utilized palladium as a reactant and not as a catalyst. The extent of methane conversion, selectivity, and reaction rates were not stated.
The Sen et al. article has been criticized in Vargaftik et al., xe2x80x9cHigh Selective Partial Oxidation of Methane to Methyl Trifluoroacetatexe2x80x9d, Journal of the Chemical Society, Chemical Communications (1990), pp. 1049-1050, to the extent that the results were not found to be reproducible. Vargaftik et al. discloses the catalytic oxo-esterification of methane to methyl trifluoroacetate with cobalt in trifluoroacetic acid but shows that palladium is not even suitable for stoichiometric methane oxidation in the process. With Pd, less than 0.1% yield of methyl trifluoroacetate based on palladium (II) trifluoroacetate was obtained.
The Vargaftik et al. article discloses that although palladium is ineffective for the conversion of methane to methyl trifluoroacetate, Co(III) can be used for this reaction. The Co(III) is said to be catalytic in the presence of oxygen. The rate of the reaction was very low, 2.5xc3x9710xe2x88x9211 mol/cc sec, (or four to five orders of magnitude away from typical commercial rates of about 10xe2x88x926 mol/cc.sec). Only four turnovers of the Co ion were disclosed. The extent of methane conversion was not stated. In addition to Co, other metals were suggested which were said to allow stoichiometric oxidation of methane to methyl trifluoroacetate in varying yields (based on amount of metal charged): Mn(30%), Cu(0.1%), and Pb(10%).
A later publication by Sen et al (xe2x80x9cHomogeneous Palladium (II) Mediated Oxidation of Methanexe2x80x9d, Platinum Metals Review, (1991), Vol 35, No. 3, pp. 126-132) discloses a catalytic system using palladium as the catalyst, peroxytrifluoroacetic acid as the oxidant, and a mixture of trifluoroacetic acid and trifluoroacetic anhydride as the solvent. The reaction rate was low (4.2xc3x9710xe2x88x929 mol/cc.sec) and only 5.3 turnovers of Pd were observed. The extent of methane conversion and selectivity were not stated.
There are several lesser routes to upgrade methane such as ammoxidation to HCN, chlorination to chloromethanes, carbon disulfide production and acetylene synthesis. Of great importance is the direct oxidation of methane to produce methanol, formaldehyde, or dimerized products, ethane and ethylene. None of these reactions, however, has yet found large-scale commercial application due to either limited catalytic activity, short catalytic life or low product selectivity. Despite the limited selectivities and yields, numerous studies have been reported on the direct partial or complete oxidation of methane.
A homogeneous system for the selective, catalytic oxidation of methane to methanol via methyl bisulfate was reported (R. A. Periana et al., International Patent Application WO 92/14738). This document describes a novel high-yield system for the catalytic conversion of methane to methanol. Homogeneous reaction takes place in concentrated sulfuric acid and is catalyzed by catalysts comprising metals such as Pd, TI, Pt, and Au with Hg being most preferred. It is said that they have achieved a yield of 43% at a methane conversion of 50% and 85% selectivity to methyl bisulfate (R. A. Periana et al., Science, vol.259, Jan. 15, 1993). The activation of methane is proposed to occur through a net electrophilic displacement reaction with mercuric bisulfate to produce methyl mercuric bisulfate. This species then decomposes to the product and the reduced species, mercurous bisulfate, in the functionalization step. In the redoxidation step, the mercurous bisulfate is oxidized by sulfuric acid, regenerating mercuric bisulfate.The problems associated with this process are that most of the disclosed catalysts are expensive, and the most preferred catalyst, Hg(II), is poisonous and environmentally damaging. Furthermore, the applied gas pressure necessary in order to achieve sufficient methanol yield is very high.
In the present invention we disclose a process which is related to that disclosed in WO 92/14738, but which employs other types of catalysts which are inexpensive, relatively non-poisonous and produces much higher yields of methanol from methane. In addition to this, to achieve the same methanol yield, the required methane gas pressure in the process according to the present invention is much lower than the pressures required in the process described by Periana et al.
In the present invention, initial chemical reaction of a hydrocarbonxe2x80x94in general a hydrocarbon in the gas phasexe2x80x94takes place in a concentrated sulfuric acid (H2SO4) medium [preferably one containing dissolved sulfur trioxide (SO3), in the present description denoted an H2SO4/SO3 medium] and is catalyzed by, in particular, substances containing iodine, titanium or chromium in various oxidation states; more generally, it would appear that species which have a standard reduction potential in the range of 0.5-1.4 volt versus the Standard Hydrogen Electrode are often well suited as catalysts in the context of the present invention.
In the case of alkanes, notably lower alkanes such as those already mentioned above, e.g. methane, an important major product of the reaction in sulfuric acid medium appears to be the corresponding alkyl bisulfate (alkyl hydrogen sulfate), e.g. methyl bisulfate in the case of methane, which may, if so desired, be isolated as an end product per se; such a process constitutes an aspect of the present invention.
Alternatively (and, particularly in the case of methane, importantly), the latter bisulfate product may subjected to a hydrolysis step (e.g. using water or another aqueous medium) to give the corresponding alkanol (alcohol), i.e. methanol in the case of methane.
The reactions of particular interest (illustrated here for methane) are shown below.
Step 1: CH4+2H2SO4xe2x86x92CH3OSO3H+2H2O+SO2xe2x80x83xe2x80x83(1)
Step 2: CH3OSO3H+H2Oxe2x86x92CH3OH+H2SO4xe2x80x83xe2x80x83(2)
Overall: CH4+H2SO4xe2x86x92CH3OH+H2O+SO2
It is envisaged that in order to reduce consumption of the concentrated sulfuric acid medium in the process, a further step could be incorporated wherein SO2 formed in the process is reoxidized [using, for example, an oxygen-containing gas (such as air or substantially pure oxygen)] to SO3 which would then react with water present in the system to give sulfuric acid. Thus, a step such as the following is contemplated:
Step 3: SO2+xc2xdO2+H2Oxe2x86x92H2SO4xe2x80x83xe2x80x83(3)
With incorporation of such a step in a process of the invention, the overall reaction of particular interest could then be written as:
Overall: CH4+xc2xdO2xe2x86x92CH3OHxe2x80x83xe2x80x83(4)
The concentrated essentially 100% sulfuric acid used as the reaction medium in the initial stage of the process plays several roles. Firstly, it is a strong acid exerting a super-acidic function in the system which contributes to retention, absorption and/or binding of the hydrocarbon in the medium during the oxidative reaction in question. Secondly, sulfuric acid acts as an oxidant, being itself reduced to sulfur dioxide. Further, it functions as a reactant in its own right, combining e.g. with alkanes such as methane to initially form a sulfate ester species, and as a solvent for the hydrocarbon and the reaction products thereof.
Thus, as already indicated to some extent, an important aspect of the present invention relates to a process for producing an alcohol from a gaseous hydrocarbonxe2x80x94such as the production of an alkanol from a corresponding alkane, especially methanol from methanexe2x80x94via oxidative reaction of the hydrocarbon in a concentrated sulfuric acid medium in the presence of a catalyst, wherein the substance added as catalyst comprises a substance selected from the group consisting of iodine, iodine compounds, titanium, titanium compounds, chromium and chromium compounds. In preferred variants of the process, the latter reaction takes place in a pressurizable reaction vessel. Stirring or other effective mixing of the reaction medium during the reaction is generally to be preferred.
In order to form the desired alcohol (e.g. alkanol) from the product or products of the reaction of the hydrocarbon in the sulfuric acid medium, the product or products (e.g. an alkyl bisulfate and/or dialkyl sulfate) in question is/are suitably subjected to an hydrolysis step, e.g. using water or an aqueous medium.
The sulfuric acid medium employed in the process preferably contains dissolved SO3 in an amount of from 0% by weight up to the solubility limit of SO3 in H2SO4 under the particular reaction conditions (temperature, pressure, etc.) employed. The SO3 content of the medium will, however, generally be in the range of 0-65% by weight (% w/w), and normally in the range of 30-65% w/w.
The reaction temperature in the sulfuric acid medium will normally preferably be in the range of 20-400xc2x0 C., often around 200xc2x0 C. With respect to pressure conditions, particularly when the gas-phase hydrocarbon in question is an lower alkane, e.g. methane, the reaction in the sulfuric acid medium will preferably take place in a pressurizable reaction vessel into which the hydrocarbon is introduced to an initial pressure in the range of 1-500 bar. It is, however, generally preferable to employ a initial pressure in the range of 20-80 bar, such as in the range of 40-70 bar.
With regard to catalytic substances of interest in relation to the present invention, the added catalyst employed is preferably one comprising:
iodine (elemental iodine) or a compound of iodine in any oxidation state (e.g. xe2x88x921 or +5);
titanium (metal, e.g. in powder form) or a compound of titanium in any oxidation state (e.g. +3 or +4), such as titanium(IV) oxide (i.e. titanium dioxide); or
chromium (metal, e.g. in powder form) or a compound of chromium in any oxidation state (e.g. +2, +3 or +6), such as a chromium (VI) compound in the form of, e.g., a chromate or dichromate (e.g. of an alkali metal such as sodium or potassium).
Among catalytic substances of the latter types, particularly suitable substances are believed to include those having a standard reduction potential in the range of 0.5-1.4 volt versus the Standard Hydrogen Electrode (SHE).
The invention thus provides, inter alia, catalytic systems in a pressurized reactor vessel, comprising a catalyst such as a compound containing iodine, titanium or chromium, an active reaction medium comprising liquid H2SO4/SO3, and a gaseous hydrocarbon as reactant. As already indicated, the resulting liquid containing oxo-esters (sulfate esters) may be hydrolyzed to produce alcohols or used to produce other compounds. The preferred parameters (temperature, pressure, SO3 content, added catalysts etc.) for such catalytic systems are those already mentioned above in the context of processes of the invention.